1. Field of the Invention
The present invention relates to an optical disc apparatus which performs reproducing of information recorded on an optical recording medium and recording of information on an optical recording medium, in particular, the present invention relates to an objective lens driving device which drives an objective lens that is arranged in the optical disc apparatus.
2. Description of Related Art
An optical recording medium such as a compact disc (hereinafter referred to as a CD), or a digital versatile disc (hereinafter referred to as a DVD) has become popular. In addition, recently, to increase amount of information to be recorded on the optical recording medium, a study about high density recording on the optical recording medium is advanced, and for example, an optical recording medium which is made for high density recording such as a Blu-ray disc (hereinafter referred to as a BD) is being put to practical use. Recording and reproducing of these optical recording media are performed utilizing an optical disc apparatus.
On the optical disc apparatus, an objective lens is arranged to condense a laser beam which is emitted from a light source on a recording surface of the optical recording medium. In case where recording or reproducing of the optical recording medium is performed utilizing the optical disc apparatus, position of the objective lens is controlled by an objective lens driving device in order that a focal point of the objective lens is always kept on the recording surface of the optical recording medium despite of waving and the like of the optical recording medium, and that a position of the beam spot which is formed by condensing of the objective lens is not displaced from track which is formed on the optical recording medium.
As above described, technology for high density recording on the optical recording medium is advanced, and based on this, high accuracy for driving accuracy of the objective lens driving device has also been required. Above all, in case where recording or reproducing of the optical recording medium is performed by the optical disc apparatus, it is necessary that the laser beam is input vertically to the recording surface of the optical recording medium from the objective lens, and it is required that a phenomenon in which the objective lens is tilted (tilting) because harmful torque is generated when the objective lens is driven by the objective lens driving device, is suppressed.
About this point study has been conventionally performed, for example, technology is reported in JP-A-2006-024266 and JP-A-2002-237067. In JP-A-2006-024266, an objective lens driving device is proposed which has a structure in that a region of a rectangular parallelepiped magnet which is arranged on the objective lens driving device, is cut two times in laterally and vertically and magnetized, and focusing coils and tracking coils are disposed in cross shape. It says that according to this, variation in direction of magnetic field and its magnitude become small and the tilting of the objective lens when it is driven can be suppressed small.
Further in JP-A-2002-237067, an objective lens driving device is proposed in that, for example, a rounded portion is set up on a driving magnet which is included in the objective lens driving device, and distributional shape of magnetic flux density on a plane where a driving coil is disposed, becomes substantially symmetry to a peak of the magnetic flux density. It says that according to this, disbalance of forces which are generated in respective portions of the driving coils, is not generated even when the objective lens is moved for focusing and tracking and any moment with respect to center of supporting of movable portion is not generated.
However, in case of the structure disclosed in JP-A-2006-024266, because two magnets which are disposed in the objective lens driving device, have quite the same dividing manner of the region but they have a contrary polarity in magnetized pattern, and it is necessary to manufacture the two magnets separately. As a result, it causes a problem that additional cost is required to manufacture the magnets.
Further, all facing regions of the two magnets that are disposed to face each other for sandwiching the objective lens, have the same magnetic polarities. Therefore, a ratio of part where direction of the magnetic field that is formed by the magnets becomes slanted without being orthogonal to the focusing coil, is increased, it is conceivable that magnetic force of the magnet can not be effectively used.
Further in case of the structure disclosed in JP-A-2002-237067, because it has a structure in that the rounded portion is set up in the driving magnets, processing step for the magnet is increased and it becomes to add load in manufacturing process, in addition, it causes a problem that manufacturing cost is increased.